Computer controlled materials transport systems are known for moving materials among various work stations of a facility. Such systems are employed, as an example, in semiconductor fabrication facilities for moving semiconductor wafers to successive work stations. In such a wafer transport system, a monorail track is routed past the various work stations and a plurality of electric vehicles are mounted on the track. The plurality of electric vehicles are moveable along the monorail track for delivering wafers to the successive work stations for processing, and for removing wafers therefrom after the requisite processing operations are completed. The track is composed of interconnected track sections which usually include one or more routing sections or modules which are operative to provide plural paths along the track.
Each vehicle includes one or more electric motors coupled to drive wheels which in turn engage the track for propelling the vehicle along the track. Electronic circuitry governed by an on board micro-controller controls operation of the vehicle in response to control signals provided from one or more central control points within the facility. It is not always possible, or desirable, to directly power the vehicle via a power rail or similar means. Therefore, a battery or battery pack is usually contained on board the vehicle for powering the motors and associated circuitry for at least a portion of vehicle operation. An ultracapacitor may also be contained on board as a supplemental power source to supply additional power during peak consumption periods. A materials transport system used for semiconductor wafer transport and other materials is described in U.S. Pat. No. 4,926,753, assigned to the same Assignee as the present invention.
Due to the power demands of an electric vehicle, the battery must be frequently recharged through either a mechanical electrical connection or inductive coupling of magnetic fields at recharging stations located at predetermined positions along the track as described in U.S. patent application and assigned to the same Assignee as the present invention. However, the batteries used to power these vehicles can be recharged only a specified number of times before they reach the end of their life cycle. In general, the life cycle of a rechargeable battery is less than that of the vehicle in which they are installed in. Because each battery replacement increases the cost of operating an electric vehicle, the cost associated with each vehicle must be increased to account for the eventual replacement of the rechargeable batteries used in it. In addition, certain types of batteries require periodic deep discharges to prevent the batteries from developing a charge "memory" that may limit the ability of the rechargeable battery to fully charge. Having to deeply discharge a battery in a vehicle being used runs the risk of stranding the vehicle without power, possibly resulting in a system blockage. This can result in a decrease in the efficient use of an electric vehicle as well.
Therefore, what is needed is a method and system that decreases the reliance of an electric vehicle on power that is supplied by a rechargeable battery by reducing the number of battery charge/discharge cycles required during a given period of vehicle operation, so as to extend the battery life.